Extrinsic chest muscles, as you can see, originate on the front of the axial skeleton and insert on the upper extremity. Generally speaking, they protract the scapula and draw the humerus toward the midline.
This gets a little more complicated with the pectoralis major as the clavicular head as it allows us to flex the humerus while protracting the scapula. We might do this when opening a door, shaking hands or reaching into the back of a low cabinet.
This means that the scapula gets trapped by pectoralis major but is the insertion point for pectoralis minor and serratus anterior. The clavicle is the origin of clavicular pec but the insertion of subclavius and trapped by serratus anterior and pectoralis minor.
Something interesting and less complicated happens when we take pectoralis major out of the picture.
The deep extrinsic chest muscles are pectoralis minor, serratus anterior and subclavius. They all originate on ribs and insert on the shoulder girdle.
When you look at the lateral view, it is easy to see how these muscles are not about moving the arm but mobilizing and stabilizing the platform on which the arm performs, the shoulder girdle. The two fan-shaped muscles, pectoralis minor and serratus anterior, grasp the corners of the scapula so it can be tilted while protracted. Subclavius acts as an assistant to protract the clavicle on which the scapula is tethered.
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